The present invention concerns a cavity member for a mold cavity structure for the production of hollow body moldings by means of injection molding.
In plastic material processing injection molding represents the most important process for the production of moldings. In the injection molding procedure the molding material in powder form or in granulate form is plasticised for example in a screw injection molding machine and then urged into the closed, generally cooled tool, for example a mold cavity structure. When the mold or the mold space provided therein is completely filled with the melt, it hardens by cooling. That generally involves a reduction in volume. That is frequently compensated by melt being further subsequently urged into the mold, from the injection cylinder. In addition the contraction is also generally taken into consideration by a suitable oversize in the mold contour. Finally the tool or the mold cavity structure is opened and the finished molding (injection molding) is removed and ejected. The tool can be closed again and a fresh working cycle can begin, with renewed injection.
By means of injection molding it is possible to produce hollow bodies which can be inflated in a later working step for example to afford bottles or canisters. Those hollow bodies are also referred to as preforms or parisons.
Mold cavity structures for the production of parisons which are intended for subsequent inflation to form PET bottles usually comprise a core, a cavity member, a base insert and a neck jaw.
In the closed condition of the mold cavity structure a mold space, the shape of which corresponds to the molding to be produced, is formed between the core on the one hand and the base insert, cavity and neck jaw on the other hand. The outside contour of the core thus forms the inside contour of the hollow body molding while the outside contour of the hollow body molding is formed by the cavity member, the base insert and the neck jaw.
The cavity member has a substantially hollow-cylindrical element. The base of the mold space is formed by the base insert which adjoins the cavity member. The neck jaw adjoins the cavity member at the side remote from the base insert.
In other words, the neck jaw, the cavity member and the base insert afford a hollow space into which the core penetrates.
In general all parts of the mold cavity structure are cooled. Therefore the cavity member has a cooling passage at the outside of the hollow-cylindrical part. Usually the cooling passage comprises a groove of spiral shape, which is introduced into the outside of the hollow-cylindrical element of the cavity member. In operation the cavity member is fitted with the remaining parts of the mold cavity structure into what is referred to as a cavity plate. The cavity plate has a corresponding recess. The cooling passage is then formed on the one hand by the spiral groove and on the other hand by the inside wall of the corresponding recess in the cavity plate, which closes the spiral groove. In most cases the cavity plate is designed to receive a multiplicity of mold cavity structures, for example 192.
Such a mold cavity structure is known for example from WO 2005/051632.
Introducing the known spiral groove into the material of the cavity member however leads to a considerable reduction in the strength or stiffness of the hollow-cylindrical element by virtue of the notch effect. In principle any deviation from a continuous cross-section leads to a notch stress which alters the strength characteristics of the component in an adverse fashion. For that reason the hollow-cylindrical element must be relatively thick-walled to prevent the cavity member breaking in operation. In addition, in the state of the art, the grooves must be provided with a rounded groove bottom in order not to excessively influence the strength characteristics. In principle however it is desirable for the cooling fluid to be passed as closely as possible to the mold space in which the molding to be produced and therefore to be quickly cooled is disposed. A thick wall and/or a rounded groove bottom are therefore rather disadvantageous.
The known spiral grooves are also relatively complicated and expensive to produce. It has also been found that, by virtue of the spiral configuration of the cooling passage, a substantial part of the cooling fluid flowing through the cooling passage does not come into contact with the cavity member by virtue of centrifugal force, and therefore also does not contribute to the cooling action. In addition the heat to be dissipated occurs substantially at the groove bottom so that a temperature gradient is formed within the cooling fluid so that the temperature of the cooling fluid decreases from the outside inwardly or from the groove bottom to the inside wall of the cavity plate recess. Accordingly because of their greater density the colder cooling fluid constituents preferably flow in the outside region of the spiral cooling passage so that it is precisely the cooling fluid flow which is particularly preferred for effective cooling that contributes only little to the cooling action.